FIGS. 21 and 22 show examples of distributed Raman amplification systems (DRA systems) in the prior art used in wavelength division multiplexing (WDM) optical fiber communication systems (refer, for example, to Non-Patent Documents 1 or 2). FIG. 21 shows a backward-pumped DRA, while FIG. 22 shows a bi-directionally-pumped DRA. This DRA system uses dispersion-shifted fiber (DSF) as a transmission path, with the typical value of the zero-dispersion wavelength (λ0) being 1540 to 1560 nm (the rated value has a slightly wider wavelength range).
In the backward-pumped DRA shown in FIG. 21, the wavelength of the WDM signal light is in the so-called L band (1575-1605 nm) (typical value), with the wavelengths of the pumping lights for backward pumping being 1470 nm and 1500 nm. In the case of the bi-directionally pumped DRA in FIG. 22, the wavelength of the signal light is in the so-called C band (1530-1560 nm) (typical value), with the wavelengths of the forward and backward pumping lights being 1420 nm and 1450 nm.
Pumping light is introduced from the inline repeaters 1-3, 2-3, 1-4, and 2-4 into the transmission path in the opposite direction to the signal light using multiplexers 14 and 24. The pumping light sources are laser diode light sources having a fiber Bragg grating (FBG) as an external mirror (laser diodes with a fiber Bragg grating, FBG pumping light sources), which are the most commonly used.
The inline repeaters 1-3, 2-3, 1-4, and 2-4 contain an erbium-doped fiber amplifier (EDFA) 16 and 26. The signal light that leaves the inline repeaters 1-3 and 1-4 on the upstream side of the DSF and propagates through the DSF undergoes distributed Raman amplification near the inline repeaters 2-3 and 2-4 on the downstream side of the DSF, and after the distributed amplification in the transmission path, undergoes the lumped amplification in EDFAs 16 and 26.
FIG. 21 shows an attempt to improve the signal-to-noise ratio (SNR) using backward-pumped DRA. In order to further improve the SNR, FIG. 22 uses a bi-directionally pumped DRA with added FBG pumping light sources 13-3 and 23-3 and multiplexers 15 and 25 as forward-pumped DRA. However, in the prior art, when applying forward-pumped DRA, the signal light wavelength band becomes limited to the C band (1530-1560 nm) (in the present invention, as shown below, signal light amplification by forward-pumped DRA is possible in the L band (1575-1605 nm)).
In the constitution shown in FIG. 21, in the case of performing forward distributed Raman amplification with the aim to further improve the SNR, similarly to backward distributed Raman amplification, amplification is carried out using pumping light with wavelengths approximately 100 nm shorter than the wavelength of the signal light. The pumping light wavelength is, for example, 1470 nm and 1500 nm (similar to the case of backward distributed Raman amplification in FIG. 21). FIG. 23 shows the Raman gain spectrum at this time. The horizontal axis of FIG. 23 represents wavelength (nm) and the vertical axis shows gain (dB). According to FIG. 23, in the L band of 1575-1605 nm, a flat gain spectrum is obtained.
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